The invention relates to an alternating phase mask for the exposure of a photosensitive layer in a photolithography process, in particular in the fabrication of large scale integrated semiconductor circuits. The phase mask has at least one T pattern structure that has at least one first and second opaque segment. The second opaque segment joins a longitudinal side of the first opaque segment and subdivides the latter into two opaque partial segments on both sides of a joining point. The phase mask has first and second transparent area segments in an alternating sequence on both sides of the partial segments of the first opaque segment and of the second opaque segment, which area segments impart a mutual phase shift by 180xc2x0 on an exposure radiation passing through them.
An alternating phase mask is disclosed for example in Published, Non-Prosecuted German Patent Application DE 199 57 542 A1, by this applicant and commonly assigned, and corresponding to U.S. patent application Ser. No. 10/158,733.
In the fabrication of integrated semiconductor circuits, mask structures assigned to the circuit elements are optically imaged in a conventional manner in light-sensitive layers on the wafer. On account of the diffraction effects, the resolution of such an imaging system is limited, and mask structures with dimensions below the reciprocal value of the resolution, the so-called critical structures, are imaged in a blurred or unsharp manner. This leads to undesirable strong correlations between the circuit elements and thus to impairment of the circuit functionality.
These difficulties can be overcome by utilizing the destructive interference effect of two closely adjacent coherent or partly coherent light beams that are phase-shifted by 180xc2x0 relative to one another. To that end, the conventional masks affected are converted into alternating phase masks in which each critical structure is provided with two-phase shifters for producing the required phase shift.
The various types of phase masks are described for example in the book titled xe2x80x9cTechnologie hochintegrierter Schaltungenxe2x80x9d [Technology of large scale integrated circuits] by D. Widmann, H. Mader and H. Friedrich, 2nd edition, Springer-Verlag, page 135 et seq. Furthermore, an extensive overview of phase mask technology is contained in the publication titled xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d by M. D. Levenson et al. in IEEE Transactions on Electron Devices 29 (1982), 1828 ff.
The use of so-called strong phase masks, which include both the alternating phase masks already mentioned and chromium-free phase masks, requires the transparent phase-shifting structures in each affected plane to be allocated to one of two phases which have a mutual phase difference of approximately 180xc2x0.
In view of the complexity of modern circuits and the requirement for two elements that affect a respective phase jump by 180xc2x0 on each critical structure, phase conflicts are unavoidable, however. A phase conflict is present precisely when the same phase is incorrectly allocated to the phase shifters on both sides of a critical structure, or when, on account of the interaction of the phase-shifting elements, the destructive interference effect occurs at an undesired point on the aforementioned light-sensitive layer.
For this problem, Published, Non-Prosecuted German Patent Application DE 100 51 134 A1 by this applicant and commonly assigned, corresponding to U.S. patent application Ser. No. 09/978,396 now U.S. Pat. No. 6,543,045, proposes an automatically proceeding method for ascertaining possible phase conflicts on alternating phase masks and for automatically eliminating the phase conflicts.
In principle, two different methods exist for resolving a phase conflict in alternating phase mask technology. One method requires the distortion of the layout in order to eliminate lithographically critical structures. However, this method is to be avoided, if possible, because it does not accord with the miniaturization process of microelectronics as controlled by Moore""s law. The second currently preferred method for resolving phase conflicts, as is disclosed in the aforementioned Published, Non-Prosecuted German Patent Application DE 100 51 134 A1 by the same applicant, consists in the artificial production of phase jumps at optimal points in the layout, so that a two-colorable set of phase shifters is produced. The phase jumps lead to dark lines on the photosensitive layer on the wafer. The dark lines are usually eliminated by a further exposure operation using a so-called trimming mask. If at least two-phase jumps are required for resolving a phase conflict, then the phase conflict is referred to as propagating.
Published, Non-Prosecuted German Patent Application DE 199 57 542 mentioned in the introduction, proposes an alternating phase mask. However, the alternating phase mask requires a trimming mask that is an alternating phase mask, that is to say requires an additional coloration.
It is accordingly an object of the invention to provide an alternating phase mask that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a double exposure technique is made available for the efficient handling of propagating T conflicts in the alternating phase mask technology.
With the foregoing and other objects in view there is provided, in accordance with the invention, an alternating phase mask for an exposure of a photosensitive layer in a photolithography process, in particular for fabricating large scale integrated semiconductor circuits. The alternating phase mask contains at least one T pattern structure having at least one first opaque segment with a longitudinal side and at least one second opaque segment. The second opaque segment joins the longitudinal side of the first opaque segment at a joining point and subdivides the first opaque segment into two opaque partial segments on both sides of the joining point. Transparent area segments, including first transparent area segments and second transparent area segments, are disposed in an alternating sequence on both sides of the two opaque partial segments of the first opaque segment and of the second opaque segment. The transparent area segments impart a mutual phase shift by 180xc2x0 on exposure radiation passing through the transparent area segments. The T pattern structure has corners and at one of the corners where the second opaque segment meets the first opaque segment, one of the first transparent area segments and one of the second transparent area segments are disposed such that the exposure radiation passing through them experiences a phase jump by 180xc2x0 there, a point producing the phase jump forming an angle with each of two affected sections of the first and second opaque area segments, and the angle substantially bisects a corner angle of the T pattern structure.
In the alternating phase mask according to the invention, the T conflict is resolved by producing a phase jump at one of the 90xc2x0 corners of the T structure. The phase jump forms an angle of 45xc2x0 with each of the two affected sections of the T-shaped structure considered. That is to say there are two equivalent solutions for each T conflict.
A major advantage is that the trimming mask for the alternating phase mask proposed, for eliminating the dark lines which remain during the first exposure operation with the alternating phase mask, is a conventional mask and consequently requires no additional coloration, in contrast to the propagation technique. A further advantage is a minimal alignment error, because the number of trimming openings is greatly reduced.
The alternating phase mask technology according to the invention can be applied to all known alternating phase masks, in particular to the alternating bright-field and dark-field phase masks.
In accordance with an added feature of the invention, two affected sections of the first and second transparent area segments are spaced apart from one another by a slot formed therebetween, and the slot separates the two affected sections of the first and second transparent area segments and forms the angle with respect to the corner of the T pattern structure.
In accordance with a further feature of the invention, a distance between the two affected sections of the first and second transparent area segments, is formed by the slot, and the slot is significantly smaller than a width of each of the two affected sections of the first and second transparent area segments. Ideally, the distance approaches zero. In other words, the distance is extremely small compared to the width of the transparent area segments.
In accordance with an additional feature of the invention, the first opaque segment and the second opaque segment of the T pattern structure have are elongated rectangular webs and meet one another at the joining point at an angle of about 90xc2x0.
In accordance with another feature of the invention, the transparent area segments in each case have a substantially rectangular shape and the two affected sections of the first and second transparent area segments form, at the slot, an end edge that runs in a manner inclined by the angle.
In accordance with another added feature of the invention, the angle is approximately equal to 45xc2x0.
With the foregoing and other objects in view there is provided, in accordance with the invention, a combination of the alternating phase mask described above, and a trimming mask for a second exposure operation following a first exposure operation with the alternating phase mask. The trimming mask contains an opaque area region encompassing the T pattern structure with the first and second opaque segments and the transparent area segments. The opaque area region has a cutout formed therein lying in a direction and at a location of the slot of the alternating phase mask and encompasses the slot. A transparent area region fills the cutout and surrounds the opaque area region.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an alternating phase mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.